1. Field of the Invention
This invention relates to a protection system for a thyristor valve used for DC power transmission e.t.c., and more particularly to a protection system for a thyristor valve which protects thyristors from voltage stress during the forward recovery period.
2. Description of the Related Art
Generally, a thyristor does not recover its forward withstand voltage until a certain time has elapsed after the end of power conduction. The "turn-OFF time" (Tq) is a guide to the time required for this recovery.
When a thyristor valve is running as an inverter, the reverse voltage period applied to the thyristor after the end of power conduction is called the "margin angle" (.gamma.). Normally, the control angle is selected to that this margin angle .gamma. is greater than turn-OFF time Tq.
When margin angle .gamma. is smaller than turn-OFF time Tq due to the generation of waveform distortion in the AC system, the thyristor cannot block a forward voltage applied following a reverse voltage, and self-fires. This phenomenon is called "commutation failure". When thyristors are connected in series, randomness will occur in turn-OFF times Tq of individual thyristors. Therefore, when margin angle .gamma. is small, there will be thyristors with and without commutation failure. This phenomenon is called "partial commutation failure". Thus, the whole circuit voltage will be applied to the thyristors without commutation failure. Therefore, when the number of thyristors with partial commutation failure is large, a forward voltage greater than the withstand voltage will be applied to the thyristors which block the voltage. This will sometimes lead to damage.
To counter this problem, in prior art, the thyristors were protected by forcibly turning them on by supplying a gate pulse to all the thyristors, if margin angle .gamma. became smaller than the maximum value of turn-OFF times Tq of the series thyristors. This is called "forced firing protection".
The details of the above-described prior art protection were well described in U.S. Pat. No. 4,377,835 issued on Mar. 22, 1983 and Canadian Patent No. 1,054,218 issued on May 8, 1979.
Such prior art forced firing protection had the following problems. That it to say, turn-OFF time Tq depends on the rising rate dv/dt of the applied forwardly rising voltage v and peak value of voltage v. However, the maximum value of turn-OFF times Tq used in prior art forced firing protection which is taken as TqO was assumed from rising rate dv/dt (several v/.mu.s) in normal operation. When an overload voltage with a larger rising rate dv/dt than in normal operation is applied to the thyristor valve from outside, due to a lightning surge or the action of an ON/OFF device, turn-OFF times of individual thyristors Tq which are taken as Tq1 became greater than turn-OFF time TqO. Turn-OFF times Tq1 has randomness for individual thyristors in the same way as turn-OFF time TqO.
Consequently, if a surge voltage penetrates into the thyristor valves between the turn-OFF Times Tq0 and Tq1 after the end of power conduction, the above forced firing protection will not operate. However, partial commutation failure will occur due to the randomness of turn-OFF times Tq1 of individual thyristors, and there is a possibility of damaging the thyristors.
Also, when a forwardly rising voltage v with a large rising rate dr/dr is applied to the thyristors, and peak value of voltage v at that time is high, all the thyristors connected in series will self-fire. Thus, there is a possibility of damage due to loss at the time of turn-ON.